I have this theory or headcanon or whatever you may call about this brother.
1. Maybe Amane is fated to die and Tsukasa made a wish to that "thing" that make it able to change fate
2. We're already shown that Tsukasa and that thing most likely swapped.
3. Amane probs slowly realize that thing is not Tsukasa.
4. Amane try to fix the clock to wish for his little brother to come back.
5. Most likely his wish actually granted in a way he doesn't expect and he forgot about it because, either he is way too traumatized or it actually his payment. Maybe that's the reason why the timekeeper in the present is weak?
6. Most likely he killed Tsukasa (that thing) in the hope of freeing his real Tsukasa, and he kills himself to be able to find Tsukasa in the supernatural world.
7. I run out of thing to talk on Amane side.
So, talking about Tsukasa, I think he's sociopath since child and only care about Amane. Since Amane always close to the death door before he make that patch with the thing, and his decision actually makes Amane healthy, he think whatever he does is the right thing. And that's why even if his action will make Amane cries and hate him (he actually like seeing it on Amane? He's in that spectrum where he love everything about a person, even their worst moment) he'll do it with the believe that's the right thing to do (for Amane).